Fix remembering selection in scrollback buffer

Commit d37d3ac1 "CompactHistoryScroll: Remove _maxLineCount + 5 lines at a time" caused a regression: addHistLine queried the number of lines in history before and after adding a line, and if it had not incremented it assumed one line entered history and another was dropped at the other end. Now, lines are dropped from history in batches, so take care of that.